So far, heat transfer techniques have been widely used for simple and expeditious printing. Allowing various images to be produced expeditiously, these heat transfer techniques have incidentally been employed for prints usually made in a small number, e.g. for preparing ID or other cards.
Where it is desired to obtain color images like photographs of face, another type of heat transfer technique is now available, making use of heat transfer films of continuous length comprising a continuous substrate film on which a number of heat transfer layers colored in yellow, magenta and cyan (and black, if necessary) are formed successively and repeatedly.
Such heat transfer sheets are generally broken down into two types, one referred to as a so-called wax type of heat transfer film in which a heat transfer layer is thermally softened and transferred onto an image-receiving material in an imagewise manner and the other a so-called sublimation type of heat transfer film in which only a dye sublimes (migrates) thermally from within a heat transfer layer onto an image receiving sheet after an imagewise pattern.
When ID or other cards are to be produced with such heat transfer films as mentioned above, the wax type of heat transfer film has the advantage of being capable of forming verbal, numerical or other images, but involves the disadvantage that such images are poor in durability, esp., rub resistance.
With the sublimation type of heat transfer film, on the other hand, it is possible to obtain gray scale images, i.e., gradation pattern, like photographs of face. Unlike those obtained with ordinary ink, however, the formed images are less lustrous for lack of any vehicle and, by the same token, are poor in durability, e.g. rub resistance.
In order to solve such problems, it has been proposed so far to laminate transparent films on the surfaces of the images. However, this is not only cumbersome to handle but gives rise to card curling as well, because the cards are laminated all over the surfaces. What is more, too thin films cannot be used in view of lamination work, thus posing a problem that the overall thickness of cards increase.
As an alternative to the above-mentioned lamination technique, it has been proposed to coat the surfaces of images with heat- or ionizing radiation-curable resins and cure them. However, this is not only troublesome to handle but also brings about a possibility that the images may be attacked by solvents in coating materials. With the heat-curable resins, there is another possibility that the dyed images may discolor or fade due to the heat used for curing.
It is therefore an object of this invention to provide a heat transfer cover film which can solve the above-mentioned problems of the prior art and so can expeditiously give excellent, curl-free images that are improved in terms of such properties as durability, esp. rub resistance, luster, color development. Another object is to provide a heat transfer process making use of such a cover film.